1. Field of the Invention
The present invention relates to a battery charger for a portable wireless telephone set and, in particular, to such a battery charger having a function for tricklingly charging the battery after the battery is charged to its normal voltage.
2. Description of the Prior Art
A portable wireless telephone set has a battery with a normal voltage as a power source for a transmitter and a receiver of the telephone set. The battery energy is consumed when the telephone set is used for telephone communication. Therefore, it is necessary that the battery is charged when the telephone set is not used for the telephone communication.
To this end, a battery charger is usually provided for the portable wireless telephone set so as to charge the battery.
A known battery charger for use in a portable wireless telephone set comprises a rapidly charging circuit and a tricklingly charging circuit which are selectively operated under control of a charge controller.
At a start of the battery charge, the rapidly charging circuit generates a high level DC current for rapidly charging the battery. Once the battery is charged to the normal voltage, the rapidly charging circuit is stopped by the charge controller and the trickling charging circuit then generates a considerably low level current for tricklingly charging the battery.
The battery charger has a charge detecting circuit for detecting whether or not the battery is charged to the normal voltage. When the charge detecting circuit detects that the battery is charged to the normal voltage, the charge detecting circuit produces a detected signal which is delivered to the charge controller. Then, the charge controller stops the rapidly charging circuit and makes the tricklingly charging circuit operate to produce the low level DC current. Accordingly, the battery is tricklingly charged by the low level DC current.
When the battery is being charged, it is provided that a power swith of the telephone set should be turned off. Therefore, the trickling charging circuit is designed to produce such a low level DC current that the battery is not excessively charged.
On the other hand, when the power switch is turned on for a stand-by or a call waiting condition, the battery energy is also consumed but at a rate lower than a condition for the telephone communication. Therefore, when the power switch of the telephone set is left in an on condition, the battery is charged to the normal voltage by the rapidly charging circuit of the battery charger but the battery energy is consumed after the battery charging operation is turned from the rapidly charging circuit to the trickling charging circuit, so that the battery is insufficiently charged. This results into a fact that the next telephone communication is impossible or limited to a reduced communication time duration by use of the telephone set.